Prior art copolymers of ethylene and longer .alpha.-olefins, i.e. olefins having about 10 or more carbon atoms, have suffered a number of disadvantages. Primarily, it has not been possible to prepare such copolymers, having sufficiently high molecular weight for most applications, using a traditional Ziegler-Natta catalyst. Generally, as the longer .alpha.-olefin comonomer content increases, the molecular weight decreases significantly. Also, the ethylene copolymers have had a very broad molecular weight distribution, as well as a broad .alpha.-olefin composition distribution. This arises from the prior art catalyst systems having a high ratio of ethylene:comonomer reactivity, as well as a low ratio of polymerization propagation to termination. As a result, the low molecular weight species have substantially higher .alpha.-olefin comonomer content and the high molecular weight species have a very low comonomer content.
It has been proposed to use certain metallocenes such as bis(cyclopentadienyl) titanium or zirconium dialkyls in combination with aluminum alkyl/water cocatalyst as a homogeneous catalyst system for the polymerization of olefins. For example: German Patent Application 2,608,863 teaches the use of a catalyst system for the polymerization of ethylene consisting of bis(cyclopentadienyl) titanium dialkyl, aluminum trialkyl and water; German Patent Application 2,608,933 teaches an ethylene polymerization catalyst system consisting of zirconium metallocenes of the general formula (cyclopentadienyl).sub.n ZrY.sub.4-n, wherein n stands for an integer in the range of 1 to 4, Y for R, CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2 and CH.sub.2 CH(AlR.sub.2).sub.2, wherein R stands for alkyl or metallo alkyl, an aluminum trialkyl cocatalyst and water; European Patent Application No. 0035242 teaches a process for preparing ethylene and atactic propylene polymers in the presence of a halogen-free Ziegler catalyst system of (1) cyclopentadienyl compound of the formula (cyclopentadienyl).sub.n MY.sub.4-n in which n is an integer from 1 to 4, M is a transition metal, especially zirconium, and Y is either hydrogen, a C.sub.1 -C.sub.5 alkyl or metallo alkyl group or a radical having the following general formula CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2, and CH.sub.2 CH(AlR.sub.2).sub.2 in which R represents a C.sub.1 -C.sub.5 alkyl or metallo alkyl group, and (2) an alumoxane; and U.S. Pat. No. 4,564,647 teaches a low pressure process for polymerizing ethylene, either alone or in combination with small amounts of other .alpha.-olefins, in the presence of a catalyst which may comprise a cyclopentadienyl compound, represented by the formula (Cp)MR.sup.2 R.sup.3 R.sup.4 wherein (Cp) represents a cyclopentadienyl group, M represents titanium, vanadium, zirconium or hafnium, and R.sup.2, R.sup.3 and R.sup.4 are each an alkyl group having from 1 to 6 carbon atoms, a cyclopentadienyl group, a halogen atom or a hydrogen atom, an alumoxane, which can be prepared by reacting trialkyl aluminum or dialkyl aluminum monohalide with water and a filler. Each of the above patents also teach that the polymerization process employing the homogeneous catalyst system is hydrogen sensitive thereby providing a means to control polymer molecular weight.
As is well known in the prior art, catalyst systems comprising a cyclopentadienyl compound, hereinafter frequently referred to as a metallocene or metallocene catalyst component, and an alumoxane offer several distinct advantages when compared to the more conventional Ziegler-type catalyst systems. For example, the cyclopentadienyl-transition metal/alumoxane catalyst systems, particularly those wherein the cyclopentadienyl compound contains at least one halogen atom, have demonstrated extremely high activity in the polymerization of .alpha.-olefins, particularly ethylene. Moreover, these catalyst systems produce relatively high yields of polymer product having a relatively narrow molecular weight distribution. However, these catalyst systems, when used to prepare copolymers of ethylene with longer .alpha.-olefins in anything more than a very minor proportion, still suffer from the drawbacks of low incorporation rates, and low molecular weights.
For many applications it is of primary importance for a polyolefin to have a high weight average molecular weight while having a relatively narrow molecular weight distribution. A high weight average molecular weight, when accompanied by a narrow molecular weight distribution, provides a polyolefin or an ethylene-lower-.alpha.-olefin copolymer with high strength properties. Traditional Ziegler-Natta catalyst systems--a transition metal compound cocatalyzed by an aluminum alkyl--are capable of producing polyolefins having a high molecular weight but a broad molecular weight distribution.
More recently a catalyst system has been developed wherein the transition metal compound has two or more cyclopentadienyl ring ligands, such transition metal compound also being referred to as a metallocene - which catalyzes the production of olefin monomers to polyolefins. Accordingly, metallocene compounds of the Group IV B metals, particularly, titanocene and zirconocene, have been utilized as the transition metal component in such "metallocene" containing catalyst system for the production of polyolefins and ethylene-.alpha.-olefin copolymers. When such metallocenes are cocatalyzed with an aluminum alkyl--as is the case with a traditional type Ziegler-Natta catalyst system--the catalytic activity of such metallocene catalyst system is generally too low to be of any commercial interest. It has since become known that such metallocenes may be cocatalyzed with an alumoxane--rather than an aluminum alkyl--to provide a metallocene catalyst system of high activity which catalyzes the production of polyolefins. The zirconium metallocene species, as cocatalyzed or activated with alumoxane are commonly more active than their hafnium or titanium analogues for the polymerization of ethylene alone or together with a lower .alpha.-olefin comonomer.
A wide variety of Group IV B transition metal compounds of the metallocene type have been named as possible candidates for an alumoxane cocatalyzed catalyst system. Hence, although bis(cyclopentadienyl) Group IV B transition metal compounds have been the most preferred and heavily investigated type metallocenes for use in metallocene/alumoxane catalyst for polyolefin production, suggestions have appeared that mono and tris(cyclopentadienyl) transition metal compounds may also be useful. See, for example, U.S. Pat. Nos. 4,522,982; 4,530,914 and 4,701,431. Such mono(cyclopentadienyl) transition metal compounds as have heretofore been suggested as candidates for a metallocene/alumoxane catalyst are mono(cyclopentadienyl) transition metal trihalides and trialkyls.
More recently International Publication No. WO 87/03887 described the use of a composition comprising a transition metal coordinated to at least one cyclopentadienyl and at least one heteroatom ligand as a metallocene type component for use in a metallocene/alumoxane catalyst system for .alpha.-olefin polymerization. The composition is broadly defined as a transition metal, preferably of Group IV B of the Periodic Table which is coordinated with at least one cyclopentadienyl ligand and one to three heteroatom ligands, the balance of the coordination requirement being satisfied with cyclopentadienyl or hydrocarbyl ligands. The metallocene/alumoxane catalyst system described is illustrated solely with reference to transition metal compounds which are bis(cyclopentadienyl) Group IV B transition metal compounds.
Therefore, a need still exists for catalyst systems that permit the production of higher molecular weight ethylene-longer-.alpha.-olefin copolymers and desirably with a narrow molecular weight distribution and a narrow composition distribution. The present invention addresses the need, then, for a polymerization process which permits the efficient and economically attractive production of high molecular weight ethylene/longer .alpha.-olefin copolymers and copolymer products.